This invention relates to circuitry for adjusting the hue of a reproduced TV image and more particularly to circuitry in a digital TV for simulating a phase change in the relative sampling points of the color mixture signals to effect such change.
Hue control in conventional (analog) TV receivers, whether automatic or manual, has traditionally been effected by phase shifting the reference signal to the color mixture signal demodulator circuits. Similarly, in the more recent digital TV receivers, hue control is accomplished by altering the phase of the sampling points, relative to burst, at which analog-to-digital conversions are made. See, for example, the article "Digital VLSI Breeds Next-Generation TV Receivers", Electronics, Aug. 11, 1981, pp. 97-103. Note that altering the sampling phase permits sampling the composite signal on any color axis desired.
Phase shifting circuits generally include circuit elements which have selectably variable parameters which are subject to drift and are not stable over extended periods of time. Thus, frequent adjustments of hue may be necessary. Digital circuits, on the other hand, are inherently stable with respect to their response characteristics. As a result, if hue correction is performed digitally, i.e. numerically, on the sampled signal, the corrections will not be subject to variations over time due to device aging, etc.